There are a number of applications for which it is desired to use fibers to deliver high power laser pulses having high output beam quality, for example, laser ignition of engines. An embodiment of such a system uses a single laser source located in the vicinity of the engine to deliver the light pulses to the combustion volumes, for example, an engine cylinder. Typical laser sources might be diode-pumped Q-switched Nd:YAG lasers. However, in typical applications where there are multiple ignition sites, for example, 6-20 engine cylinders, a distributed approach with a single laser and fiber optic delivery is preferred. Fiber delivery (using a single laser) increases safety, allows the laser to be located away from the high temperature conditions near the cylinder and, especially for multi-cylinder engines, should enable lower cost multiplexed solutions when compared to mounting a laser on each cylinder. Thus, the fiber delivery approach can potentially provide implementations that are simpler, less expensive, safer, and more reliable in the vibrating and hot engine environment; however fiber delivery has been difficult.
Although tight focusing of the fiber output beam is required for spark formation, the use of conventional single-mode (SM) fibers having small core size is problematic for several reasons: the maximum power that such cores can carry is generally insufficient, the tolerance for misalignment at the launch becomes exceedingly difficult to maintain in practical systems, and diffraction and lens aberration may limit focused spot sizes (to several microns) making it difficult to sufficiently de-magnify the fiber output. Although silica (breakdown) damage limits for 1064 nm nanosecond pulses in bulk silica can be as high as 475 GW/cm2, safe operating limits in multimode fibers are generally much lower, for example, IBD,Si≅1−5 GW/cm2, where IBD,Si is the intensity for breakdown in silica. Because of the large disparity in the breakdown level of air (It is to be noted that combusting gases are primarily air) relative to the fiber material, that is, IBD,Air/IBD,Si≅100-300, spark formation in air (without damage to the fiber) requires the light exiting the fiber be imaged at the spark location with linear demagnification of ≳10-20, which has been very difficult to obtain. More generally, even neglecting challenges of energy delivery, the use of small core single-mode fibers (core ˜5-10 μm) is limiting because diffraction and aberration prevent focusing the output to needed spot-sizes of ≲1 μm, while for typical large core fibers the multi-mode (high M2) output precludes the needed demagnification.
Similar considerations apply for laser-induced breakdown spectroscopy (LIBS), laser drilling, cutting, and welding applications, as well as for surgical applications in medicine, all of which can benefit from fiber delivery of high power light pulses with high spatial beam quality (low M2). In contrast to telecommunications, such applications typically require pulse delivery over relatively short distances (between about 1 m and approximately 20 m).
Hollow core fibers can be used to deliver high power pulses to form sparks in air; however, their versatility is limited by bending-induced degradation of modal quality.